1. Field of the Invention
The present invention relates to a light source, and more particularly to an apparatus for stabilizing a wavelength of a light source.
2. Description of the Related Art
Recent development of public networks, such as the Internet, etc. requires ultrahigh-speed optical communication, and an optical transmission method using wavelength division multiplexing (WDM) has been widely used. In order to improve the performance of a WDM optical transmission system, it is necessary to increase the number of channels contained in a designated bandwidth. Intervals between wavelengths can be sufficiently reduced by stabilizing the wavelengths of a light source. An operating wavelength of a distributed feedback laser diode (DFB), which is generally used as a light source of the WDM optical communication system, varies according to variations in surrounding environment, e.g., temperature, and is further deteriorated because of the time duration at it is used. Thus, DFBs are not suitable for being set to a desired wavelength for extended periods.
In one conventional method for stabilizing the wavelength of a laser diode, a thin-film type optical filter or Fabry-Perot etalon filter is used that selectively transmits or reflects light emitted from the laser diode according to the wavelengths of the light, and then senses and stabilizes the light. Since the above conventional method employs the thin-film type or crystal type optical filter, it is difficult to manufacture a wavelength stabilizing apparatus integrated into an optical fiber or a planar lightwave circuit (PLC) and the wavelength stabilizing apparatus has a high loss and failure rate
In order to solve the above problems, another method is proposed for using an optical fiber Bragg grating (FBG) formed in an optical fiber. Conventionally, the FBG includes an optical fiber having a core provided with a grating engraved therein and a clad surrounding the core, and a planar lightwave circuit having a core (or, a core layer or a waveguide) provided with a grating engraved therein and a clad (or, a cladding layer) surrounding the core.
FIG. 1 is a block diagram of a conventional wavelength stabilizing apparatus of a light source. The wavelength stabilizing apparatus 100 comprises a light source (LS) 110, an optical isolator (ISO) 120, an optical coupler type filter 130, a grating 140, a first optical detector (OD1) 150, a second optical detector (OD2) 160, a signal processor 170, and a driver 180.
The light source 110 outputs light having a known wavelength through both surfaces thereof so that light 112 outputted from the light source 110 through the front surface is used in optical communication and light 114 outputted from the light source 110 through the rear surface is used for stabilizing the wavelength. A semiconductor light source, such as a laser diode, may be used as the light source 110.
The optical isolator 120 is disposed between the rear surface of the light source 110 and the optical coupler type filter 130, and passes light 114 outputted from the rear surface of the light source 110 and isolates light inputted in the opposite direction.
The optical coupler type filter 130 is disposed between the optical isolator 120 and the grating 140, and includes a first waveguide 132 provided with first and second terminals (A1 and A2), a second waveguide 134 provided with third and fourth terminals (A3 and A4), and a coupling region 137 where the first and second waveguides 132 and 134 are adjacent to each other. The first terminal (A1) is connected to the optical isolator 120, the second terminal (A2) is a free end, the third terminal (A3) is connected to the second optical detector 160, and the fourth terminal (A4) is connected to the first optical detector 150. The optical coupler type filter 130 is divided into three regions, i.e., the coupling region 137, a first connecting region 136 formed between the coupling region 137 and the first or third terminal (A1 or A3), and a second connecting region 138 formed between the coupling region 137 and the second or fourth terminal (A2 or A4). The light 114 inputted to the first terminal (A1) passes through the coupling region 137, thereby being transferred from the first waveguide 132 to the second waveguide 134 (this is referred to as “coupling”). The transferred light 114 is then provided to the fourth terminal (A4).
The grating 140 is disposed between the fourth terminal (A4) of the optical coupler type filter 130 and the second optical detector 160, and serves to transmit a partial light 115 (hereinafter, referred to as “transmitted light) having the designated wavelength of the light 114 and to reflect the remainder lights 116 and 117, i.e., light have a wavelength other than the designated wavelength. For example, the grating 140 is manufactured by irradiating ultraviolet rays on a core made of a photosensitive material through a phase mask. The lights 116 and 117 reflected by the grating 140 are inputted to the fourth terminal (A4), and pass through the coupling region 137. The light 116 (hereinafter, referred to as “extracted light”) is provided to the third terminal (A3), and the light 117 (hereinafter, referred to as “recurrent light”) is provided to the first terminal (A1). The branch rate of the reflected lights 116 and 117 is determined by the multiplication of a length of the coupling region 137 and a coupling coefficient (Kab) representing a degree of coupling of the first and second waveguides 132 and 134.
The first optical detector 150 detects the extracted light 116 inputted from the third terminal (A3) as an electric signal (hereinafter, referred to as a “first electric signal”), and the second optical detector 160 detects the transmitted light 115 inputted from the grating 140 as an electric signal (hereinafter, referred to as a “second electric signal”). Each of the first and second optical detectors 150 and 160 may include a photo diode.
The signal processor 170 determines the wavelengths of the lights 112 and 114 outputted from the light source 110 based on powers of the first and second electric signals inputted from the first and second optical detectors 150 and 160. That is, in case that a difference between the power of the extracted light 116 and the power of the transmitted power 115 is less than a designated value, it is determined that the light source 110 normally outputs the light having a designated wavelength (that is, the light source 110 is operating normally). On the other hand, in case that the difference between the power of the extracted light 116 and the power of the transmitted power 115 is not less than the designated value, it is determined that the light source 110 is operating abnormally. In case that the light source 110 is operating abnormally, the signal processor 170 outputs a control signal to the driver 180 to drive light source 110 from the abnormal operation.
The driver 180 stabilizes the output wavelength of the light source 110 based on the control signal. For example, the driver 180 increases or decreases an operating temperature of the light source 110 to stablize the wavelength.
As described above, the optical isolator 120 prevents the output characteristics of the light source 110 from being deteriorated due to the influence of the input of the recurrent light 117 to the light source 110, that is reflected by the grating 140.
However, the optical isolator 120 is an expensive product, thus increasing the production costs of the wavelength stabilizing apparatus 100. Further, the optical isolator 120 increases the volume of the wavelength stabilizing apparatus 100. Moreover, since it is difficult to manufacture the optical isolator 120 in a planar lightwave circuit type, the wavelength stabilizing apparatus 100 cannot be integrated into a planar lightwave circuit.
Accordingly, there is a need in the industry for a lower-cost apparatus for wavelength stabilizing laser outputs.